Method for Manufacturing a Product from a Flexibly Rolled Strip Material

ABSTRACT

A method for manufacturing a product from a flexibly rolled strip material includes the steps of: providing a strip material made from sheet steel; flexibly rolling the strip material such that a variable thickness is produced along the length of the strip material; electrolytically coating the strip material with a metallic coating material containing at least 93% of zinc by mass after the flexible rolling; heat treating at temperatures above 350° C. and below a solidus line of the coating material after the electrolytic coating; working a blank from the flexibly rolled strip material; and hot forming the blank.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation patent application of U.S. patentapplication Ser. No. 14/078,025, filed Nov. 12, 2013, which claimspriority from German Patent Application No. 10 2012 110 972.9, filedNov. 14, 2012. The disclosures of both applications are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method for manufacturing coated steel sheetsmade from a flexibly rolled strip material. The steel sheet should beprotected against corrosion by means of the coating.

Different methods for coating components made from steel with a zinc orzinc alloy layer are known, like hot galvanization (hot-dipgalvanization) or galvanic (electrolytic) galvanization. Hotgalvanization means plating of steel parts with a solid metallic zinccoating by means of dipping of the pretreated steel parts into a melt ofliquid zinc. Galvanic galvanization is carried out by dipping theworkpieces into a zinc electrolyte. Electrodes of zinc serve, because oftheir less precious metal, as a “sacrificial anode”. The workpiece to begalvanized serves as a cathode, because of which the coating is alsocharacterized as a cathodic corrosion protection.

From DE 10 2007 013 739 B3 a method for the flexible rolling of coatedsteel strips is known. A hot or cold strip is electrolytically coatedand subsequently flexibly rolled, wherein the coated steel stripsreceive different sheet thicknesses along the length. The coating isadjusted to the sheet thickness after flexible rolling or to the rollingpressure during the flexible rolling. For this, the coating is formedvaryingly thick.

From DE 10 2009 051 673 B3 a method for manufacturing steel strips witha cathodic corrosion protection layer is known. For this, the steelstrip is hot rolled, subsequently cold rolled and is electrolyticallygalvanized. After the electrolytic galvanization, the steel strip isheat treated in a bell-type annealing furnace at temperatures from 250°C. to 350° C. for a time of 4 to 48 hours, whereby a zinc-iron layer isproduced.

From DE 10 2007 019 196 A1 a method for producing flexibly rolled stripmaterial with a cathodic corrosion layer is known. The method comprisesthe steps of providing a rolled strip as a hot or cold strip with acathodic corrosion layer, and flexibly cold rolling of the coated rolledstrip with a rolling gap adjustable during the rolling process.

From DE 601 19 826 T2 a method for achieving a workpiece with very highmechanical properties is known, which starting from a steel sheet stripis formed by means of deep-drawing. The workpiece is hot rolled andcoated with a metallic alloy made from zinc. For this, the sheet is cutto size, heated up to a temperature of 800° C. to 1200° C. andsubsequently a hot deep drawing process is carried out. Then, the sheetexcesses, necessary for the deep drawing process, are removed by meansof cutting.

SUMMARY OF THE INVENTION

The present invention is based on the object to provide a method formanufacturing coated steel sheets from a flexibly rolled strip material,which offers an especially good corrosion protection.

A first solution consists of a method for manufacturing a product from aflexibly rolled strip material comprising the steps: providing a stripmaterial made from sheet steel, flexible rolling of the strip material,wherein a variable thickness is produced along the length of the stripmaterial, electrolytic coating with a metallic coating material, whichcontains at least 93% by mass of zinc, wherein the electrolytic coatingis carried out after the flexible rolling, heat treatment attemperatures above 350° C. and below a solidus line of the coatingmaterial, wherein the heat treatment is carried out after theelectrolytic coating, working a blank from the flexibly rolled stripmaterial, and cold or hot forming of the blank.

A second solution is a method for manufacturing a product from aflexibly rolled strip material comprising the steps: providing a stripmaterial from sheet steel, flexible rolling of the strip material,wherein a variable thickness is produced along the length of the stripmaterial, electrolytic coating with a metallic coating material, whichat least contains zinc and iron, working a blank from the flexiblyrolled strip material and cold or hot forming of the blank.

An advantage of the two above named methods is, that the electrolyticcoating is carried out after the flexible rolling. Thus, it is achieved,that the deposited coating has a constant thickness along the length ofthe flexibly rolled strip material. Insofar also the areas of the stripmaterial, which are stronger rolled-out, have a layer thickness, whichreliably protects against corrosion. Altogether the process time formanufacturing the product can be shortened and less coating material isnecessary, which again has an advantageous effect on the manufacturingcosts.

A flexibly rolled product is, in connection with the present invention,understood to be a steel strip with varying thickness as well as arectangular blank or a form-cut (profile cut), respectively, which isproduced from a flexibly rolled steel strip by means of mechanicalcutting or laser cutting. As strip material for the flexible rolling, ahot strip or cold strip can be used, wherein these terms should beunderstood in the sense of the technical terminology. A hot strip ishere seen to be a rolling steel finished product (steel strip), which isproduced by means of rolling after preliminary heating. A cold strip ishere meant to be a cold rolled steel strip (flat steel), at which thelast thickness reduction is carried out by means of rolling without apreceding heating. The strip material which is provided for being rolledcan also be referred to as band material.

In both of the above named solutions it is understood, that between theindividual method steps, further steps could be interposed. For example,after the flexible rolling, a strip straightening can be provided. Theworking of the blanks from the strip material can be carried out beforeor after the electrolytic coating. Conceptually, “working a blank from astrip” is supposed to include, that the sheet blank can be stamped fromthe strip material, which means an edge remains at the strip, which isnot further used, as well as, that a simple cutting of the stripmaterial into partial pieces can be carried out, especially by means ofa cutting process. Working a blank from a strip can also be referred toas producing a blank from a strip.

In the first solution, a coating consisting at least of 93% by mass ofzinc is deposited on the strip material, wherein the proportion of zincmay especially be larger than 95% by mass, 97% by mass, or 99% by massand can even be 100% (pure zinc coating). For the electrolytic coating,anodes made from pure zinc or from zinc and other alloy elements areused, which, during feeding of current, deposit metal ions on theelectrolyte. The zinc ions and possible ions of further alloy elementsare deposited as atoms on the strip material, which is connected as acathode, and form a coating. In a deposition of a coating with a highproportion of zinc of more than 93% by mass as it is provided in thefirst solution, the following heat treatment leads in an advantageousmanner to an alloy formation between the deposited zinc and the ironcontained in the strip material, so that altogether a zinc-iron coatingis produced.

In the second solution, from the start, a zinc-iron-alloy layer isproduced by means of electrolytic deposition. The proportions of zincand iron are preferably selected such that at least one of the followingconditions is valid: the alloy layer contains at least 5% by mass ofiron, the alloy layer contains at a maximum 80% by mass of iron, thealloy layer contains at a minimum 20% by mass of zinc and/or the alloylayer contains at a maximum 95% by mass of zinc. It is especiallyadvantageous when the proportions of zinc and iron are selected suchthat in the deposited state, at least partially δ1-phase, especiallyδ1-phase and Γ-phase, or only intermetallic Γ-phase is present. This is,for example, achieved with an iron proportion of 10% by mass to 30% bymass percent or a zinc proportion of 70% by mass to 90% by mass, whereinthe addition of further alloy elements is not excluded. In thisembodiment, a subsequent heat treatment is not necessary, as the coatingitself already contains zinc and iron. The zinc and iron atoms arearranged at a distance of few nanometers from each other so thatespecially short diffusion paths are produced. It can, however, beunderstood that also with an electrolytic deposition of a zinc-ironalloy, the named heat treatment can be carried out. By means of theshort diffusion paths, a very short heat treatment is sufficient, forexample by means of induction heating. Altogether, by means of the namedmethod process, a shortening of the process time can be achieved in anadvantageous manner.

The method according to the second solution can be carried out accordingto a first possibility without heat treatment after the electrolyticcoating and before forming. According to a second possibility of thesecond solution, a heat treatment at a temperature range above 350° C.and below the melting temperature of the coating material (solidus line)can be provided as a further step after the electrolytic coating. Thesolidus line marks in the finite state diagram for the coating materialthat line, below which only solid phase is present. Above the solidusline the coating material is at least present partially as melt.

With progressing heating time, the iron proportion in the coatingincreases, as iron atoms diffuse from the base material into the coatingmaterial. Because of the increasing iron proportion in the coating, theheat treatment temperature can then be increased, without reaching thesolidus line or exceeding it. This is possible with suitable processcontrol up to a temperature of 781° C. The possibility of thetemperature increase during the heat treatment is obviously also validfor the first solution. The temperature can be step-wise or continuouslyincreased with increasing iron proportion.

The liquidus line marks in the finite state diagram for the coatingmaterial that line, below which a two-phase or multi-phase range,solid-liquid, is present. Above the liquidus line, the coating materialis in the liquid form. The lower limit of the two-phase range ischaracterized as the solidus line. The temperature of the solidus linedepends on the proportional composition of the alloy. For pure zinc, thesolidus line lies at 419.5° C., for a zinc-iron alloy it is maximal 782°C., insofar as still parts of Γ-phase are present. With a correspondingproportion of iron it is, thus, possible, to electrolytically coat theflexibly rolled strip material in a full hard (hard as rolled) conditionand subsequently to carry out a heat treatment at a relatively hightemperature of more than 500° C. up to maximal 782° C., without that aliquid phase is produced.

A heat treatment in a temperature range of 500° C. up to 782° C. is,furthermore, suitable to carry out a re-crystallization annealing, sothat the produced material is especially suited for an indirect hotforming. An otherwise necessary re-crystallization annealing can, thus,be omitted after the flexible rolling and before the coating. Forexample, in the first named solution with the use of pure zinc (coatingmaterial 100% zinc), the heat treatment process can be started at anannealing temperature of 380° C. and, with increasing iron proportionsdue to diffusion processes, can then be step-wise increased up to atemperature of maximal 781° C.

For both solutions it applies, that the coating material can alsocontain further alloy elements, like for example manganese, chromium,silicon or molybdenum. Independent of the type and number of alloyelements, a feature of the invention is the temperature control for thepurpose of forming the zinc-iron alloy layer. The respective alloytemperature is selected such, that the solidus line of the coatingmaterial in the composition, currently present during the process, isreached or exceeded at no point of time of the alloy formation of thebinary zinc-iron phase diagram or of a layer structure, containing morethan two alloy elements, respectively. The alloy is thus formed by solidphase diffusion.

During the heat treatment, a diffusion of iron from the coated materialinto the metallic coating takes place. In this case, zinc of the coatingconverts into a zinc-iron alloy, which offers a cathodic corrosionprotection. The stated temperature range above 350° C. and below thesolidus line is especially advantageous, as the diffusion takes placerelatively quickly. Because of the iron content, the affinity to soldercracking of the coating is reduced, so that the fatigue limit of thecomponent is increased.

The phase conversion can be achieved, as mentioned above, according to afirst possibility by means of inductive heating. This process method isespecially suitable in an electrolytic deposition of zinc and iron, ashere short diffusion paths are present, so that a short heat treatmentcan lead already to the required phase conversion. According to a secondpossibility, the heat treatment can be carried out by annealing in abell-type annealing furnace. This annealing is especially suitable forthe electrolytic deposition of pure zinc. Preferably, during theannealing in an annealing furnace a holding time of 10 to 80 hours,preferably 30 to 60 hours, is provided, so that sufficient time isavailable, so that by means of diffusion a zinc-iron alloy is produced.The holding time (dwell time) characterizes preferably the whole time,in which the blanks or the strip material is heat treated, and can alsocomprise a heating-, holding, and cooling phase. A further possibilityis the conductive heating, but other technically possible heat treatmentmethods are obviously not excluded.

As a further method step it can be provided before the electrolyticcoating that the strip material is coated with an intermediate layer. Asintermediate layer, especially a nickel or aluminum containing layer canbe used. These are layers, which contain at least partially nickel oraluminum, which also includes a pure nickel layer or a pure aluminumlayer. The nickel layer forms an additional protection of the surfaceand improves the adhesion of the coating, subsequently deposited andcontaining zinc. The nickel coating can be formed, for example, byelectrolytic deposition or deposition without a current from an externalsource. It is obvious, that other materials are not excluded for theintermediate layer. For example, also a coating containing manganese orchromium can be used. Manganese and chromium have both a cubic latticeand have a good solubility in iron, which has advantageous effects onthe alloy behavior.

According to a possible embodiment, the strip material can be providedwith a scaling protection after the electrolytic coating. This isespecially applicable, when the austenitization for a later hot formingis not carried out in an inert gas atmosphere. Scaling are mainly oxidiccorrosion products, produced during the reaction of metallic materialsin air or other oxygen containing gases at a high temperature. Thedeposition of the scaling protection layer can be carried out byspraying or rolling, respectively coating. Besides the protectionagainst oxidation, a further advantage of the scaling protection layeris, that the surface has a high quality. Especially, before a latervanishing of the sheet, no cleaning treatment like shot-blasting isnecessary. Furthermore, because of the scaling protection, the frictionvalue is positively influenced during the hot forming as well as theheat absorption behavior. A further advantage of the scaling protectionis, that the adhesion of the cathodic corrosion protection layerarranged below is improved. Furthermore, a widening of thetemperature-time window in the frame of the austenitization is possible,for example by means of alloy formation of the scaling protectionmaterial with the below arranged layer. The scaling protection can bedeposited before or after the heat treatment carried out below thesolidus line.

At a suitable position of the process, blanks or form cuts are producedfrom the flexibly rolled strip material, which can be carried out bymeans of mechanically cutting or by means of laser cutting. Blanks areunderstood to be especially rectangular sheet plates, which are cut fromthe strip material. Form cuts means in particular sheet elements, cutfrom the strip material, which outer contour is already adapted to theform of the final product. Predominantly the term blanks is useduniformly for rectangular blanks as well as for form cuts. Themanufacture of blanks can be carried out before or after theelectrolytic coating and if necessary before or after the deposition ofa scaling protection.

According to a possible process embodiment which is valid for bothsolutions, the sheet blanks are hot formed. Hot forming means formingprocesses in which the workpieces are heated up to a temperature in therange of the hot forming, before being formed. The heating is carriedout in a suitable heating device, for example in a furnace. The hotforming can be carried out according to a first possibility as anindirect process, which comprises the partial steps cold pre-forming ofthe blanks to a pre-formed component, subsequent heating at least ofpartial areas of the cold pre-formed component up to an austenitizationtemperature, as well as subsequent hot forming for producing the finalcontour of the product. Austenitization temperature is understood to bea temperature range, in which at least a partial austenitization(structural conditions in the two-phase area of ferrite and austenite)is present. Furthermore, it is also possible, to austenitize onlypartial areas of the blank, to enable for example a partial hardening.The hot forming can be carried out according to a second possibilityalso as a direct process, which is characterized in that at leastpartial areas of the blank are directly heated to austenitizationtemperature and are subsequently hot formed to the required finalcontour in one step. An earlier (cold) pre-forming does not take placehere. Also during the direct process, a partial hardening can beachieved by means of austenitization of partial areas. For bothprocesses it is valid, that a hardening of partial areas of thecomponent is also possible by means of varyingly tempered tools or byusing several tool materials, which enable different cooling velocities.In the latter case, the whole blank or the whole component can becompletely austenitized.

According to a process embodiment, which is valid for both hot formingprocesses, the coating material at the point of time of initiating thehot forming is preferably in the solid state, i.e. the temperature hascooled down to an area below the solidus line of the coating material.After the hot forming, the iron content in the boundary layer is below80% by mass, preferably below 60% by mass, especially preferred below30% by mass.

According to an alternative process embodiment, which is in principlevalid for both above named solutions, the sheet blanks can also be coldformed. Cold forming are forming processes, in which the blank is notspecifically heated before forming. The forming thus takes place at roomtemperature, the blanks are heated by the dissipation of the fed energy.Cold forming is especially used as a process for forming soft car bodysteels.

The solution of the above named object is further a sheet blank madefrom flexibly rolled sheet steel, which is electrolytically coated afterthe flexible rolling with a metallic coating and is hot formed after thecoating. Thus, the above named advantages of a constant layer thicknessalong the length of the flexibly rolled strip or of the blanks producedtherefrom is achieved. The blanks are produced according to one or moreof the above named method steps, so that concerning the steps and theadvantages connected therewith it is referred to the above description.

Following, preferred embodiments are described by using the figures. Itshows

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a method according to the invention as a flow chart schematicallyin a first embodiment,

FIG. 2 a method according to the invention as a flow chart schematicallyin a second embodiment,

FIG. 3 a method according to the invention as a flow chart schematicallyin a third embodiment, and

FIG. 4 a zinc-iron-phase diagram.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a method according to the invention for manufacturing aproduct from a flexibly rolled strip material 2 according to a firstprocessing embodiment. In the method step V1, the strip material 2,which is wound onto a coil 3 in the starting condition, is worked byrolling, more particularly by means of flexible rolling. For this, thestrip material 2, which before the flexible rolling has a moresubstantially constant sheet thickness along the length, is rolled byrolls 4, 5 such, that a variable sheet thickness is produced inlongitudinal direction of the rolling direction. During rolling, theprocess is monitored and controlled, wherein the data, determined from asheet thickness measurement 6, are used as input signals for adjustingthe rolls 4, 5. After the flexible rolling, the strip material 2 hasvarying thicknesses in rolling direction. The strip material 2 is woundagain to a coil 3 after the flexible rolling, so that it can betransferred to the next method step.

After the flexible rolling, the strip material 2 is smoothed in themethod step V2, which is carried out in a strip straightening device 7.The method step of smoothing is optional and can be omitted.

After the flexible rolling (V1) or the smoothing (V2), respectively, thestrip material 2 is provided with an anticorrosive coating in the methodstep V3. For this, the strip material 2 passes through an electrolyticstrip coating device 8. It is visible, that the strip coating isproduced in through-feed method, this means, that the strip material 2is wound from the coil 3, passes through the coating device 8 and afterthe coating process is again wound to a coil 3. This method process isespecially advantageous, as the handling expenditure is small fordepositing the anticorrosive coating onto the strip material 2 and theprocess velocity is high. The strip coating device 8 comprises animmersion tank 9, which is filled with an electrolytic liquid 10,through which the strip material 2 runs. Guiding of the strip material 2is achieved by means of sets of rolls 11, 12.

The electrolytic coating is achieved in the present method embodimentwith a metallic coating material, which contains at least 93% by masszinc. Because of the high zinc content, an especially good resistance tocorrosion is achieved. It is understandable, that the zinc proportioncould also be higher, for example larger than 95% by mass, especiallylarger than 97% by mass and can even be 100% by mass (pure zinc). Forthe coating process for example anodes made from zinc can be used, whichrelease during a current feed zinc ions to the electrolyte. The zincions are deposited as zinc atoms and form a zinc layer on the stripmaterial 2, which is connected as a cathode. Alternatively, also inertanodes and a zinc electrolyte can be used.

Besides the mentioned zinc proportion, the coating can still containfurther alloying elements, as for example aluminum, chromium, manganese,molybdenum, silicon. The proportion of the added alloying elements, ifnecessary, are less than 7% by mass. Manganese has a good solubility iniron, which has an advantageous effect on the alloy formation duringheating.

After the electrolytic coating (V3), the strip material 2, wound to acoil 3, is heat treated in method step V4. The heat treatment can becarried out in principle in any technically suitable manner, for examplein an annealing furnace such as a bell-type annealing furnace or also bymeans of inductive heating, to only name two methods for example. In thepresent case the heat treatment is shown in a furnace 13.

The heat treatment is carried out at temperatures larger than 350° C.and below the solidus line of the coating material. The temperatureprofile of the solidus line depends on the proportional composition ofthe alloy. At the temperature within the stated range, a diffusion ofiron is triggered into the zinc layer, so that with progressing holdingtime of the heat source a diffusion layer is produced.

During the heat treatment a diffusing of iron from the to be coatedstrip material into the metallic coating takes place. In this case, zincof the coating converts into a zinc-iron alloy, which offers a cathodiccorrosion protection system. Because the temperature range is above 350°C. and below the solidus line, the diffusion takes place relativelyquickly. The holding time for the heat treatment in an annealing furnaceis preferably 10 to 80 hours, preferably 30 to 60 hours, so thatsufficient time is available, so that a zinc-iron alloy is formed bydiffusion.

A further effect of the heat treatment is, that hardenings of thematerial, produced during the rolling, are reduced or disappear, so thatthe rolled strip material 2 takes up again a higher ductility andelasticity. The strip material can be easier further processed in thefollowing method steps, wherein furthermore the material properties ofthe to be manufactured finished product can be positively influenced.

After the heat treatment (V4) the individual sheet blanks 20 are workedfrom the strip material 2 in the next method step V5. The working of thesheet blanks 20 from the strip material 2 takes place preferably bymeans of stamping or cutting. Depending on the shape of the to bemanufactured sheet blanks 20, these can be stamped from the stripmaterial 2 as a shape cut, wherein a strip of the strip materialremains, which is not further used, or the strip material 2 can simplybe cut to length into partial pieces. A sheet blank 20, worked from thestrip material 2, which also could be characterized as three-dimensionalsheet blanks (3D-TRB), is shown schematically.

After producing the blanks 20 from the strip material 2, a forming ofthe blanks 20 to the required finished product takes place in methodstep V5. According to a first possibility the blanks 20 are hot formedor according to a second possibility cold formed.

The hot forming can be carried out as a direct or indirect process.During the direct process, the blanks 20 are heated to theaustenitization temperature before the forming, which heating can forexample be done by means of induction or in a furnace. Austenitizationtemperature is, in this case, a temperature range, in which at least apartial austenitization (structure in the binary phase region ferriteand austenite) are present. However, also partial areas of the blankscan be austenitized, to enable for example a partial hardening. Afterthe heating to the austenitization temperature, the heated blank isformed in a shape-giving tool 14 (forming tool) and at the same time iscooled with a high cooling velocity, wherein the component 20 receivesits final profile and is hardened at the same time.

During the indirect hot forming, the blanks 20 are pre-formed before theaustenitization. The pre-forming takes place in the cold condition ofthe blank, i.e. without prior heating. During the pre-forming thecomponent receives its profile, which however still does not correspondto the final shape, however this is approximated. Then, after thepre-forming an austenitization and hot forming takes place, like duringthe direct process, wherein the component receives its final shape andis hardened.

The steel material should, insofar as a hot forming (direct or indirect)is provided, contain a proportion of carbon of at least 0.1% by mass upto 0.35% by mass.

Alternatively to the hot forming as a shape giving process, the blankscan also be cold formed. The cold forming is especially suitable forsoft body steels or components, which do not have special requirementsin view of strength. During the cold forming, the blanks are formed atroom temperature.

A special feature of the method according to the invention is, that theelectrolytic coating (V3) is carried out after the flexible rolling(V1). The coating deposited on the strip material 2 has a constantthickness along the length, i.e. independent of the respective thicknessof the strip material 2. Also the areas, which have been rolled moreintensely to a smaller thickness, have a sufficient thick coating, whichprotects reliably against corrosion. A further special feature is thestep of heat treatment after the electrolytic coating at a temperaturerange between 350° C. and below the solidus line of the coatingmaterial. Because of the heat treatment, zinc diffuses from the coatinginto the base material and iron from the base material into the coating.With increasing iron proportion in the coating, the temperature duringthe heat treatment can slowly be increased because of the displacementof the solidus line towards higher temperatures. A zinc-iron alloy isproduced as coating, which withstands also higher temperatures of asubsequent hot forming process if needed and offers a reliable corrosionprotection.

It is understood, that the method sequence according to the inventioncan also be modified. For example, between the named steps, alsointermediate steps, not shown here separately, can be provided. Forexample, the strip material can be provided with an intermediate layer,especially a nickel, aluminum, or manganese layer, before theelectrolytic coating. This intermediate layer forms an additionalprotection of the surface and improves the adhesion capability of thesubsequently deposited coating containing zinc. It can also be provided,that the strip material or the blanks manufactured therefrom, areprovided with a scale protection after the electrolytic coating (V3) andbefore or after the heat treatment (V4). This is especially advisable,when the austenitization for a later heat forming does not take place inan inert atmosphere. The deposition of the scale protection can becarried out by means of spraying or calendar coating. Besides theprotection against oxidization, a further advantage of the scaleprotection layer is, that the surface has a high quality. Furthermore,the friction coefficient during the hot forming as well as the heatabsorption behavior is positively influenced by the scale protection. Afurther advantage of the scale protection is, that the adhesion of thecathodic anti-corrosion layer arranged below is improved. Furthermore, awidening of the temperature-time-window during the austenitization ispossible, for example by means of alloy formation of the scaleprotection material with the layer arranged below. An example for thisis aluminum fins in a scale protection lacquer.

Further it is understood, that the processing according to the inventioncan also be modified concerning the sequence of the carried out steps.For example, the working of blanks can also be carried out at anotherpoint, for example before the electrolytic coating or if necessarybefore or after the deposition of a scale protection.

FIG. 2 shows a method according to the invention for manufacturing asheet blank from a strip material 2 according to a second processingembodiment. This corresponds in wide parts to the method of FIG. 1, sothat in view of the similarities it is referred to the abovedescription. At the same time, the same or modified components or stepsare provided with the same reference numerals as in FIG. 1. In thefollowing particularly the differences of the present methods aredescribed.

The method steps V1 (rolling), V2 (strip straightening), V5 (stamping)and V6 (forming) are identical to the corresponding method steps V1, V2,V5 and V6 of FIG. 1.

A first difference of the present embodiment to the method of FIG. 1 isthe method step V3 of the electrolytic coating. In the present methodprocessing of FIG. 2, the strip material is coated with a metalliccoating material, which contains at least zinc and iron. Thezinc-iron-alloy layer is produced by the electrolytic deposition of azinc-iron-layer. The proportions of zinc and iron are in this caseselected according to an advantageous method processing such, that thealloy layer contains at least 5 mass percent and/or at a maximum 80 masspercent, or that the alloy layer contains at least 20 mass percentand/or at a maximum 95 mass percent of zinc.

Especially advantageous is, when the proportions of zinc and iron areselected such, that in the deposited state at least partially δ1,especially δ1-phase and Γ-phase, or only intermetallic Γ-phase ispresent. For this, for example a proportion of iron in the coating canbe selected between 10% by mass to 30% by mass, or a proportion of zincof 70% by mass to 90% by mass. With these proportions at least partiallyan intermetallic phase is formed in the deposited state.

It is advantageous for carrying-out a direct hot forming, when thecontent of Γ-phase is relative high and the content of δ1-phase is assmall as possible. To prevent solder fissuring or cracking, the meltingtemperature of the coating for the hot forming should be relative high.With the increase of the proportion of iron and thus with increasingproportion of Γ-phase, the solidus line is displaced in the binary phasediagram of zinc-iron (see FIG. 4) towards higher temperatures.

After the electrolytic coating (V3) blanks are worked from the stripmaterial 2 in method step V5, wherein it is obvious, that the blankscould also be cut-out in a modified method processing before thecoating.

A further feature of the present method sequence of FIG. 2 is, thatbetween the step of coating (V3) and the step of forming (V6) nointerconnected heat treatment is carried out below the solidustemperature. The method of FIG. 2 is thus time-wise especially short.

The subsequently carried-out step of forming corresponds to that of FIG.1, so that concerning this it is referred to the above description. Theblank 20 can be cold or hot formed (directly or indirectly).

It is understood, that also in the present method sequencemodifications, especially additional intermediate steps or subsequentmethod steps, can be carried out. It is, concerning this, referred tothe above description for preventing repetitions.

FIG. 3 shows a method according to the invention for manufacturing asheet blank from a strip material 2 according to a third methodprocessing embodiment. This corresponds essentially to a combination ofthe methods of FIGS. 1 and 2, so that concerning the similarities it isreferred to the above description. At the same time, the same ormodified components or steps are provided with the same referencenumerals.

Steps V1 (rolling), V2 (strip straightening), V3 (electrolytic coating),V5 (stamping) and V6 (forming) are identical to the corresponding methodsteps of FIG. 2. The only difference to the method of FIG. 2 is, thatafter the electrolytic coating (V3) a heat treatment is carried out inmethod step V4, as in the method of FIG. 1.

As in the method processing of FIG. 1, also in the present methodprocessing of FIG. 4, the special feature is the temperature control forforming a zinc-iron-alloy layer. The respective alloy temperature isselected during the heat treatment (V4) such, that at no point of timeof the formation of the alloy, the solidus line of the binaryzinc-iron-phase diagram (compare with FIG. 4) or the solidus line of alayer structure, consisting of more than two alloy elements, is reachedor exceeded.

An example for such a layer structure would be for example a ternaryalloy from zinc, iron and manganese, wherein the manganese stems fromthe steel substrate and reaches by means of the diffusion during theabove named heating into the electrolytically deposited zinc layer orzinc-iron-alloy layer and does not form part of an electrolyticdeposition. Instead of manganese it is also possible, that for examplechromium or aluminum or silicon or molybdenum diffuses into theelectrolytically deposited layer. It is understood, that for the coatingalso steel alloy elements can be provided, which have not been named upto now and which are suitable, to diffuse by the above named heatingprocess into the electrolytic deposited layer.

Also in the present method sequence modifications, especially additionalintermediate steps or subsequent method steps, can be carried out.Concerning this, for preventing repetitions it is referred to the abovedescription.

FIG. 4 shows the phase diagram for zinc-iron. On the x-axis, theproportions of iron (Fe) and zinc (Zn) are shown, respectively. In thiscase, on the left edge, a material with 100% by mass iron and 0% by masszinc is present, while at the right edge, inversely 0% by mass iron and100% by mass zinc is present. Between the edges, respectively, thepercentaged composition, which is stated on the x-axis, is found. Scharacterizes the molten mass, α and γ are iron-zinc-mixed crystalsystems (rich in iron), ζ and δ or δ1 and Γ are intermetallic phases,and η is a zinc-iron mixed crystal (rich in zinc).

In the following, by means of the zinc-iron phase diagram, differentpossibilities of the electrolytic deposition according to one of themethods according to the invention are exemplary described.

During the deposition of a pure zinc layer, as it can be produced in amethod processing of FIG. 1, at the beginning an alloying temperatureabove 350° C. and below the melting temperature (solidus line) of 419.5°C. is selected, for example 400° C. At this temperature, a diffusion ofiron into the zinc layer takes place, so that with continuing holdingtime during the heat treatment (V4) a diffusion layer is formed, forexample a 6-phase. The further temperature processing is such, that therespective temperature is always below the solidus line of the binaryzinc-iron-phase diagram.

During an electrolytic deposition of a coating, which already containsiron in the zinc layer, as it can be produced in a method processing ofFIG. 3, the starting temperature can be selected above the meltingtemperature of pure zinc. For example, in a composition of theelectrolytic deposited layer of 85% by mass zinc and 15% by mass iron, astarting temperature of 600° C. can be selected. This temperature liesin fact above the melting temperature of zinc, however below the solidusline of the two-phase-range F+61.

For an electrolytic deposition of a zinc-iron layer, which consists of60% by mass of zinc and of 40% by mass iron, a starting temperaturesmaller than 782° C. is possible. An increase above this temperature isonly then possible, when the layer is enriched during a following heattreatment so far with iron, that only an austenitic iron mixed crystalwould be present (for example 70% by mass percent iron and 850° C.).

The type of heat treatment is, as above described, not prescribed. Forexample, it can be an inductive heating or a heating in an annealingfurnace or a heating by means of contact with a hot body, for example athick steel plate, which delivers its heat to the blank or the profilecut.

In a special embodiment of the invention, an electrolytic zinc-ironalloy with an iron proportion of 8% by mass to 12% by mass is provided.In this case, it is a composition, as it is used for steels with aso-called “galvannealed” coating. The advantage of this composition isthat the elements zinc and iron have a distance in the range ofnanometers so that a drawn-out diffusion treatment can be waived.Rather, by means of a short heat treatment in the method step V4, anintermetallic δ1-phase can be produced from an electrolytic depositedzinc-iron alloy with an iron proportion of 8% by mass to 12% by mass.Such a composition can be used for the cold forming as well as for thehot forming.

In a further special embodiment of the invention, an electrolyticzinc-iron alloy is deposited, which stoichiometric compositioncorresponds to the Γ-phase. Alternatively, this composition can also bereached by a deposition of a zinc-iron layer with a low iron proportionand a subsequent heat treatment, at which end the Γ-phase is present.This layer starts only to melt at a temperature of 782° C., so that thislayer is especially suitable for the hot forming, as in this case theformation of a melting phase can be restricted or can be prevented bymeans of stabilizing the layer with elements from the steel substrate asmanganese (ternary system iron-zinc-manganese).

In a further embodiment, which is also provided for the hot forming(V6), a layer is electrolytically deposited, which itself is not presentin a molten state even during the heating to the maximum austenitizingtemperature for the hot forming (for example at 900° C.). Such a coatingwould for example have a composition of 20 weight percent of zinc and 80weight percent of iron. In this case, it is an iron based alloy of thebinary iron-zinc system.

Altogether with the method according to the invention products with areliable cathodic corrosion protection can be manufactured, which areespecially suitable for a hot forming process. By means of the at leastas far as possible prevention of the formation of a liquid phase in thecoating during the process, the susceptibility to cracking of the solderof the product is minimized in an advantageous manner.

REFERENCE NUMERALS LIST

-   -   2 strip material    -   3 coil    -   4 rolls    -   5 rolls    -   6 thickness control    -   7 smoothing device    -   8 coating device    -   9 immersion tank    -   10 electrolyte    -   11 set of rolls    -   12 set of rolls    -   13 furnace    -   14 molding tool    -   20 blank    -   V1-V6 method steps

What is claimed is:
 1. A method for manufacturing a product from a flexibly rolled strip material comprising the steps of: providing a strip material made from hardenable sheet steel, flexible rolling the strip material, wherein a variable thickness is produced along the length of the strip material, electrolytically coating the strip material with a metallic coating material that contains at least 93% by mass of zinc, wherein the electrolytic coating is carried out after the flexible rolling, heat treating the strip material at temperatures above 350° C. and below a solidus line of the coating material, wherein the heat treatment is carried out after the electrolytic coating, working a blank from the flexibly rolled strip material, and hot forming the blank.
 2. The method according to claim 1 wherein the metallic coating material has a minimum of 5% by mass of iron and a maximum of 7% by mass of iron.
 3. The method according to claim 2 wherein the proportions of zinc and iron in the coating material are selected such that at least partially δ1-phase is present after the step of electrolytically coating the strip of material.
 4. The method according to claim 2 wherein the temperature is increased during the heat treatment.
 5. The method according to claim 2 wherein the heat treatment is carried out inductively or by means of annealing in a bell-type annealing furnace, wherein the annealing is carried out with a holding time of 10 hours to 80 hours.
 6. The method according to claim 1 wherein before the electrolytically coating step, the strip material is coated with an intermediate layer.
 7. The method according to claim 6 wherein the intermediate layer contains nickel or aluminum or manganese.
 8. The method according to claim 1 wherein after the electrolytically coating step, a scaling prevention is deposited.
 9. The method according to claim 1 wherein the hot forming includes the steps of: cold pre-forming of the blank to a cold pre-formed component, heating at least a partial area of the cold pre-formed component up to austenitization temperature, and hot post-forming of the cold pre-formed component for producing a final contour.
 10. The method according to claim 1 wherein the hot forming step includes the steps of: heating at least a partial area of the blank up to the austenitization temperature, and hot forming of the blank for producing a final contour.
 11. The method according to claim 1 wherein at a point of time when initiating the hot forming step, the coating material is in a solid state. 